What is the best way to stop mould forming on the inside of window frames?

Hi What you need are trickle vents fitted in the windows These are the open and shut vents you see at the top of windows These will ventilate all the windows and stop the condensation forming which in turn causes the mould to form Hope this helps Rgs Steve

Your right with the ventilation. Also it depends if the right sealants have been used.

As above ventilation is definately required, what material are the frames made from? wood? metal? ventilation will help and the right sealents,however if there is an underlying problem in the integrity of the frame (rot, holes)the problem will return and keep returning, check the frames for any sign of rot/holes, also check above the frame, is there damp there? any moisture from above? Bleach will eventually damage the seal. Had the same problems on my house all the window frames were very old.